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ADU (tiny house) Battery ? Grid Tie ? Hybrid System ? VS Rewiring ?

nifty-stuff.com

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California made some really big changes to state laws regarding ADU's (Accessory Dwelling Unit), making it MUCH easier for people to add a small house onto their existing residential property.

We're in the process of evaluating a few companies (sonderpods and habitat adu) to have a pre-manufactured 750 sq. ft. unit placed onto our yard.

These units are 100% electric, including a HeatPump hot water heater & mini-split HVAC.

... but it will cost $8,000 to run electricity (240 v) to this new "house". :(

Well, here's the BIG Question:
I'm wondering if I should allocate that $8k from running a whole new line, and put it towards a battery/inverter system instead?

Another idea: Could I "trickle charge" a battery system with a 120 v, 20 amp circuit that is already close to the unit (so, still avoiding the $8k for the new service line) to a battery / inverter system that could then run the entire house? For example, it would trickle-charge 24/7 on 120v, but then be able to provide massive power bursts throughout the day.

I think there would be lots of ways to do this, but seeing the new videos for the BLUETTI EP500 got me thinking, "Hey, I could buy two of these units and a handful of solar panels for less than running a new electrical line... and BONUS, it would provide backup power during a blackout."

Really looking forward to your guys' thoughts!
 
I'm gonna lob a rock into this pond.

This is a self-contained unit for most of it (can't say about water & sewage handling) which likely varies where it is installed. Depending on Roof Design it may be quite possible to put solar on it. Alternately a Solar Pagoda or similar could be put into the yard to provide a Solar Rack and shade spot as well.

Next, I would consider how much power the unit will actually use for a typical day and does it need 240VAC or can 120VAC do ?
As yourself IF you really need 240V for anything ?

* Hot Water "Tanks", dryers, electric coil stoves are Power Pigs. Energy Efficient Appliances will save you more than the cost of equipment to generate & store power to use them and this pays back fairly quickly. Careful consideration of what appliances & devices are used is important. Even in the Mini-Split AC/Heater systems there is a huge difference in energy efficiency between brands & models and some are shockingly terrible.

Here is the trick. Once you can install some solar panels along with something like a Growatt or MPP-Solar All-In-One, plus a respectable battery bank, then you could use the VAC Input on the AIO Unit as a secondary charger source that can be wired to a 120V/20A circuit fed from your main house. Solar could handle a good part of the charging and if it runs short the AIO could be programmed to start charging using the Grid Power from that circuit. Many can be even p[rogrammed to only do so during specified times of day, often this is used for TOU Rate Shifting.

Even a simple 2000W Solar Rack (8x250W or 6x325W) can deliver 7-9kWh on a reasonably good day in Cali. You could easily accomplish this for less than $8K USD, especially if you DIY the battery packs.

As far as permits and such, Cali is WEIRD. But as long as you are NOT pushing anything to the grid, it should be a heap load simpler to get done. You'd only be pulling power to feed a Charger and that's it. You would still need a panel shutdown system and a few things for general common sense safety but nothing too crazy.

You can easily get panels New/Used for cheap from companies like Santansolar and even complete "kits" from others that could do it all.
 
I think the big question is that you will not be able to get a permit unless you show sewer, water and electric service to the ADU. The long term economics do not favor being off grid when the grid connection is close by. However California does require new construction be Net Zero but I don't know if that applies to ADUs. At any rate solar can have a good payback.
 
Does your ADU permit require separate metered service or just a power connection to its subpanel? How far is the ADU from your main service panel and how many Amps is that service panel.
 
I'm gonna lob a rock into this pond.
... and I appreciate you doing so!

This is a self-contained unit for most of it (can't say about water & sewage handling) which likely varies where it is installed. Depending on Roof Design it may be quite possible to put solar on it. Alternately a Solar Pagoda or similar could be put into the yard to provide a Solar Rack and shade spot as well.
Roof size, location, orientation, etc. would be ideal for solar if I decide to go that route. One item I neglected in my post: I actually recently more-than-doubled the size of the array on my main house with the expectation of adding the ADU... which is one of the reasons I was thinking a "trickle-charge" from the main house to the ADU would be an interesting idea.

Next, I would consider how much power the unit will actually use for a typical day and does it need 240VAC or can 120VAC do ?
As yourself IF you really need 240V for anything ?
Power consumption: Ya, that's going to be tricky since all tenants (we'll be renting it out)... but my best-guess is probably around 15 - 20 kWh.
Voltage: That's something I need to dig into, but from what I understand, at a minimum the Hot Water Heater and Oven/Range will need 240v (and probably the dryer).

Here is the trick. Once you can install some solar panels along with something like a Growatt or MPP-Solar All-In-One, plus a respectable battery bank, then you could use the VAC Input on the AIO Unit as a secondary charger source that can be wired to a 120V/20A circuit fed from your main house. Solar could handle a good part of the charging and if it runs short the AIO could be programmed to start charging using the Grid Power from that circuit. Many can be even p[rogrammed to only do so during specified times of day, often this is used for TOU Rate Shifting.

Even a simple 2000W Solar Rack (8x250W or 6x325W) can deliver 7-9kWh on a reasonably good day in Cali. You could easily accomplish this for less than $8K USD, especially if you DIY the battery packs.
Now we're talkin'! This is the stuff that excites me and makes me want to dive-in and give this a try!!!! :cool:??

As far as permits and such, Cali is WEIRD. But as long as you are NOT pushing anything to the grid, it should be a heap load simpler to get done. You'd only be pulling power to feed a Charger and that's it. You would still need a panel shutdown system and a few things for general common sense safety but nothing too crazy.
Agreed... very weird! I'm already net-metering my house, but I think for simplicity, safety, etc. I'd probably either ONLY pull a trickle from the main house to charge the system, and/or *MAYBE* add panels that cover the absolute bare-minimum baseline usage (time to jump into some spreadsheets) ;)

@Steve_S thanks again for all this great info and ideas! You've got me really thinking about this!!!!
 
I think the big question is that you will not be able to get a permit unless you show sewer, water and electric service to the ADU. The long term economics do not favor being off grid when the grid connection is close by. However California does require new construction be Net Zero but I don't know if that applies to ADUs. At any rate solar can have a good payback.
Ya, SUPER great points! Sewer and water will be easy, but I don't know how the city/county permits would feel about this type of electrical setup and/or how the ADU company would feel about doing something so... um... "novel".

This company is building the ADU 100% "title 24 compliant"... which is why everything is electric, heat-pumps, etc. In most of their builds, they would require solar added to the unit to be title 24 compliant... but since we already have overkill output on our roof, we've been told we're ok with not having to add (more) to the ADU.

Does your ADU permit require separate metered service or just a power connection to its subpanel?
Fortunately the ADU does NOT require sep. metered service. The current (see what I did there?) plan is to add another sub-panel off of an existing service panel.

How far is the ADU from your main service panel and how many Amps is that service panel.
So, our setup is... um... unique. ;) I'm gonna get into the weeds, so you may be sorry you asked ;)

We have the main house, an "inlaw-setup" (basically a "Conversion JADU" built 17 years ago), and then we're adding this new ADU.

The house has a SUPER old main service with two dual 100-amp breakers. (our solar taps into the top circuit):
MAIN-SERVICE-Panel-IMG_20200724_151100.jpg

That super old main service feeds TWO sub-panels... one for each half of the house.

The solar taps into the "top one".

Then the "bottom one" has a branch off of it into a sub-sub panel for the conversion JADU.

... an YES, all this stuff has been 100% permitted over the years! :D

The general contractor working with the ADU company wants to split off the "top" circuit to feed the new ADU (similar to what was done for the older JADU with the other panel).

... and I'm thinking to just avoid that entire thing (cost and complexity) by just slapping in a huge battery/inverter system that is trickle-charged by solar and/or the main house.
 
Another Stoned skipped across the pond...
Now 14:19 hours and I am very north compared to you (Algonquin Park Ontario Canada)
My 2000W Solar System has already generated 10.kWh, while charging my batteries, running my home AND charging individual LFP cells I am setting up for my final 24V/280AH pack.

Couple of TIPS on Energy Saving
1) Inductive Cooktops are the most efficient of electric cooktops.
2) On-Demand Hot Water is more efficient than the rest, even if electric. *
2a) If using a Tank with extra insulation & supplementally powered by a Solar Dump Load, these can be quite efficient as well - longer tech discussion to do that.
A Hot Water tank is like having your car sit in the driveway all day long because you "may" decide to go for a drive. They run 7/24/365, typically kick on at least 2x per hour to "maintain temps" whether it has been used or not. They decrease in efficiency throughout their use as the elements age and collect buildup from the water. The harder the water (more mineral content) the worse it is. The amount of Hot Water people use per day is not really that much when you add up the Gallons actually used, especially when you are conscious of it.
3) NOT all dryers are equal. Some can be quite efficient but Sun & Wind is the cheapest of the lot...
4) Ovens are a tricky thing for energy efficiency.


Screen from my Midnite Classic-200 Solar Charge Controller. snapped just now.
1614885671996.png
 
I'm doing lots of digging! ;)

To toss back some love to the owner of this site... Will makes this look so easy!

He makes it look SOOOO EASY! ;)

I mean... it's pretty tempting to put a few of these Growatt all-in-one systems together (like you mentioned in your post above @Steve_S ) with enough batteries, etc. and IN THEORY it should power everything, no?

Let's say that my tenants will use about 15 kWh per day at a max pull of 10kW. Wouldn't this suffice?
  • 3-4 of the Growatt 3000 watt inverters in parallel (they say they support up to 6 in parallel connection) (about $3,500)
  • 3-4 of the 48V LAMA - LIFEPO4 5.3KWH BigBattery units (about $8,000)
... that's the cost without solar.

I guess if I'm constantly trickle-charging the system I may not need as many batteries (which are clearly the high $$ item).

... then there's the whole "instead of more batteries, buy more solar panels" idea.

So many things to think about... so many variables, many unknown!
 
I guess I'm just really trying to wrap my head around the long-term pros / cons of simply tying into the main house's power vs. building out this entire (mostly) self-contained system.

One important bit is the longevity / reliability of the pieces. Once I'm tied to the grid, that should be all-set for many years no problem.

... but batteries (even LifePo4), inverters, charge controllers, etc. etc. etc. have limited lifespan, so I may be "better off" just dumping the $8k into running the new electrical line :(
 
... and I'm thinking to just avoid that entire thing (cost and complexity) by just slapping in a huge battery/inverter system that is trickle-charged by solar and/or the main house.
I don't know the distance but $8,000 seems like a lot for a trench to the ADU and some conduit, wires and a breaker. He probably has to dig a trench for the sewer and while the excavator us there he could dig the trench for electical. Does it go accross sidewalk or driveway? If so that might be the reason for the cost. BTW while the trench is open throw in another conduit for cable or Internet.
 
I don't know the distance but $8,000 seems like a lot for a trench to the ADU and some conduit, wires and a breaker. He probably has to dig a trench for the sewer and while the excavator us there he could dig the trench for electical. Does it go accross sidewalk or driveway? If so that might be the reason for the cost. BTW while the trench is open throw in another conduit for cable or Internet.
Valid question. Here's the Scope of Work:
  • 10 feet of trenched underground to new panel location as per plan
  • 74 feet will run through attic to subpanel within the house
  • Install 2 8-foot ground rods
  • Install new #4 ground wire at ground rod
  • Install 84 feet of new #2 copper wire service line for ADU
  • Make final connect at pod
  • Complete all building inspections for service panel
  • Contractor supply all material
  • Contractor to complete all inspections
  • Install New 125AMP Panel inside main house hallway New Panel ($1,800)
  • Total Sum for Electrical $8,100
There are clearly parts that I could bid-out and probably get done cheaper through a separate sub-contractor, but this is the general contractor for the entire ADU prep and installation, and is one part of his pricing for the entire job.
 
The more I dig into this, the more I think it may not be the best option (at least not until battery prices come way down) :(

So, in addition to all the costs, I'll also have to replace the batteries and/or inverters when they die, and monitor the system to ensure a good life span.

That being the case, it might just make the most sense to pay the $8k to connect to the grid :(
 
LOL, Okay Nifty, I can feel the adrenaline rushing in ya... slow down a second there and breath slowly. ;-)
Appreciate that Growatt, MPP etc also make bigger units than those 3K ones you're contemplating in your excitement, 6, 10 & 12 Kw exist too. btw, I remember being like that too even though it was a while ago, ;-)

Again, if the Unit can be self-sustainable or mostly so energy-wise, with a possible backup line to your main house for battery charging IF required then even better. As Ampster suggested, if there is trenching involved, toss in extra wires in conduit for Cable, Telco, Power to Unit and Power FROM Unit *.

Power To/From I would suggest wiring two cables capable of 240/200A. They can be brought to a box, and broken out from there. Even if you are only using 120V, you can use 1 leg from the main house on one wire, it just has to be wired that way. If pulling a 240V line for charging, even better & simpler. Why 2 Lines ? The Unit could be needed in the event of an emergency, or possibly for future expansion/integration with the main house. Why 200A ? Running wire that is over-capacity is fine but you'll only have to pull it once, so if you only run 30A or 100A and it is capable of 200, no problem. The one wire that is "Live" has to be fitted properly, obviously. The second wire just leave a big coil loop with the ends taped off dead-headed if needed, if you are not setting up a bi-directional switch over right away.

Renters are never energy conservative unless they are paying for it. Provide free energy and they will abuse it, sorry but it is what it is. One exception may be if family, grandparents or in-law suite or something like that. If renters bring their own appliances, be pretty sure they will not be efficient, older, or just low priced and "good enough". Again, sorry but... Ask anyone who rents out.

Permits & Inspections in California and all the "red-tape" stuff is insane and costly from everything I've heard/read. People are diy'ing it all the time & so they have to be able to get through the paper & fees. I guess that depends on what is going on too... if a full grid connection then it's one thing, if not grid connected then it's another. In my suggestion, the ADU is only connected to your house as outgoing to act as battery charge backup and not feed anything to grid. The second line could run to a sub-panel in main house to support a few circuits but would have to be on an ATS system so it never touches grid power. If grid ON, No Connect, If Grid OFF Connect. For things like Fridge, Freezer, a light circuit - basics.

Sorry to toss gravel in the pond... lol
 
@Steve_S it's always a pleasure to read your fun and informative posts and replies. GREATLY appreciated!

Again, if the Unit can be self-sustainable or mostly so energy-wise, with a possible backup line to your main house for battery charging
Wow, this is part of the value-proposition I hadn't even considered... in the case our entire property goes down in power (which can happen from time to time in California), I could... in theory, run a power line from the new ADU... even via fat extension cords, to the fridges and any other emergency items, in the main house and other JADU.

Renters are never energy conservative unless they are paying for it. Provide free energy and they will abuse it, sorry but it is what it is. One exception may be if family, grandparents or in-law suite or something like that. If renters bring their own appliances, be pretty sure they will not be efficient, older, or just low priced and "good enough". Again, sorry but... Ask anyone who rents out.
Ya, and this is clearly going to be exacerbated by the fact that we're including all utilities in the rent. I'm hoping that the month-to-month and rental agreement caveat of something like ".... if usage exceeds bla bla bla average, additional fees apply bla bla bla".

Permits & Inspections in California and all the "red-tape" stuff is insane and costly from everything I've heard/read.
Very valid. In fact, it would be pretty frustrating to do all the homework on this only to find out that nobody in the city, inspections, permits, etc. is going to let me do it.
 
Invest some money into a kWh meter so you can either charge your tenant (where allowed) or at least budget them.
Agreed! I've got the Emporia Vue Energy Monitor on my main panel and really like it! I'll definitely be adding monitoring to the new circuit as well, and will have some language in the lease about usage, overage, etc. Fortunately at the end of the day we do month-to-month, so if someone is using way too much power, we can just increase the rent the next month and they can decide if they want to stay or not.
 
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